The present invention relates to AC generators for vehicles, more particularly to a multiphase, particularly three-phase, AC generator provided with a so-called Lundell type rotor comprising an exciting coil enclosed by a pair of finger magnetic poles having a plurality of pole pieces, respectively. Such a generator is chiefly installed on vehicles, with the rotor driven by an engine thereof.
As is well known in the art, three-phase AC generators (hereinafter simply referred to as ACG) as described above generally have a basic construction such as shown in FIG. 1. In FIG. 1, reference numeral 1 designates an armature iron core of a cylindrical shape, and reference numeral 2 designates three-phase armature windings wound about the iron core 1. The armature assembly comprising these elements is supported within two housings H. Reference numeral 3 designates a three-phase full wave rectifier unit contained in one of the two housings H, for converting the output of the armature windings 2 into DC current.
A Lundell type rotor R is rotatably supported within the housings H and driven, for example, by a crankshaft of an engine (not shown) through a belt and a pulley Q. The rotor R comprises a pair of finger magnetic poles 4N and 4S having a plurality of pole pieces, respectively, which are opposed to the armature iron core 1 through a small gap G, and a cylindrical exciting coil 5 enclosed by the magnetic poles 4N and 4S for energizing them to opposite polarities. Reference character W designates a cooling fan.
When exciting current is applied to the exciting coil 5, a pair of finger magnetic poles 4N and 4S are energized to opposite polarities (N and S show each polarity) and a magnetic flux .PHI. flows toward the armature iron core 1.
In general, in an ACG of this type, magnetic poles 4N and 4S having equilateral trapezoidal surfaces opposite to the armature iron core 1 are arranged alternately by an equal pitch with each other, as shown in FIG. 2 where the magnetic structure is developed into a plane. In FIG. 2, reference numeral 11 designates the tooth portions of the armature iron core 1.
Conventionally, it is considered proper and electromagnetically efficient to make the length L of the base of the equilateral trapezium slightly shorter than the pole pitch and to make the length l of the side opposite to the base equal to the width of the tooth portions 11 of the armature iron core 1, to provide a one slot skew. It is also considered that the magnetic sound vibration can be decreased by setting the inclining rate 1 slot skew.
However, in an ACG as described above, there often occurs a problem in that the magnetic sound generated under loaded operation becomes an abnormal sound in the relatively low speed region. The magnetic sound is generated by the vibration of the housings, engine block, and the like, which is caused by the fact that when the finger magnetic poles are excited and rotated to alternately pass the tooth portions of the armature iron core, a torque variation is produced and acts on the armature iron core supported within the housing as an exciting force.
The frequency of the torque variation has a fundamental frequency T.multidot.N, which is the product of the total number of the tooth portions of the armature iron core T (generally 36 or 42) and the number of revolutions N, and the integral order harmonics of the fundamental frequency T.multidot.N. These are the frequency components of the magnetic sound which is generated in the well known ACG.
Accordingly, an object of the present invention is to provide a polyphase AC generator having an improved rotor pole construction by which magnetic sound is greatly reduced without reduction in output.
Another object of the present invention is to provide a polyphase AC generator having an improved rotor pole by deviating pole pieces relative to an armature iron core and cancelling torque variation which causes magnetic noise.